1. Field of the Invention
The present invention relates to a pneumatic radial tire; more particularly, it pertains to a heavy duty radial tire having bead portions with improved durability, which may be used on rough roads.
2. Description of the Related Art
In conventional heavy duty pneumatic radial tires for use on rough roads, separation of the carcass from its surrounding rubber material often occurs near the turn-up end of the carcass in the bead portion, and is recognized to be an important and serious problem. Hence, for such a radial tire, it is known to realize a bead portion with a desired rigidity, by arranging therein a carcass of at least one ply of steel cords oriented substantially at 90.degree. to the circumferential direction of the tire, such that the carcass extends about a bead core from an axially inner side to outer side thereof. A reinforcing layer in the form of a steel cord chafer is arranged on axially outer side of the turn-up portion of the carcass which, in turn, is arranged on the axially outer side of the bead core. Furthermore, a rubber stiffener stock is arranged between the turn-up portion of the carcass and the carcass body, i.e. that portion of the carcass which is on the axially inner side of the bead core.
When, however, radial tires are used on a rough road under a heavy load condition, side walls of the tire are subjected to a severe deflective deformation due to impact forces applied particularly when the tires ride over stones. It has been found that, during such a deformation, the durability of the bead portions is adversely affected by discontinuous location in terms of rigidity, which exists between the side wall with flexibility and bead portion with rigidity. That is, undesirable separation tends to occur at the turn-up end of the carcass or at the radially outer end of the steel cord chafer, both situated near the above-mentioned discontinuous location.
Various proposals have been made heretofore, in an effort to mitigate the occurrence of separation, including additional provision of textile chafer on the outer side of the steel cord chafer, modification in rubber material arranged near the radially outer end of the carcass turn-up portion or of the steel cord chafer, and the introduction of a stiffener in which flexible rubber and rigid rubber are combined. Still, however, none of these attempts proved to achieve a sufficient solution.